Switching regulators have been implemented as an efficient mechanism for providing a regulated output in power supplies. One such type of regulator is known as a switching supply circuit, which controls the flow of power to a load by controlling the “ON” and “OFF” duty-cycle of one or more high-side switches coupled to the load.
Many different classes of switching supplies exist today. One type of switching supply circuit is known as a synchronous switching supply circuit. In a synchronous switching supply circuit, an inductor is used to maintain current flow that is switched from two separate sources. The two sources can include a high-side switch, such as a high-side field-effect transistor (FET), and a low-side switch, such as a low-side FET. After the high-side FET is deactivated, the low-side FET becomes activated. The low-side FET thus conducts current from ground to the inductor because magnetic power stored in the inductor dissipates to force current through the inductor by changing the voltage of the inductor source node to negative relative to ground. In this way, current continuously flows through the inductor, even at times when the high-side switch is deactivated.
It is desirable in the design of switching supplies to ensure that the output of the switching supply circuit is properly regulated. For example, if a load at the output of the switching supply circuit changes, it may be necessary to change the switching operation, such as by adjusting the switching duty-cycle, to regulate the output voltage to a relatively constant level. Regulation is typically accomplished through feedback control, by either a voltage feedback technique, in which the output voltage of the switching supply is monitored, or a current feedback technique, in which both the output voltage and the inductor current are monitored. The current feedback technique can monitor the inductor current by connecting a current sense resistor in series with the output inductor. However, a resistor connected in series with the output inductor can result in a degradation of the performance efficiency of the switching supply circuit.
Another way to accomplish the current feedback technique is by employing inductor direct current resistance (DCR) sensing, which is determining the inductor current by measuring the voltage drop across the parasitic resistance of the inductor. However, DCR sensing has several pitfalls, such as requiring external temperature compensation, dealing with large DCR tolerances that limit overall accuracy, the need to have additional routing from the integrated circuit to power components and the inability to utilize minimum DCR inductors to preserve signal integrity.